arch/ppc64: Pass cbmem_top to ramstage via calling argument

This avoids the need for a platform specific implementation of
cbmem_top.

HOW TO TEST? There is no serial console for the qemu target...

Change-Id: I68aa09a46786eba37c009c5f08642445805b08eb
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36276
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Marty E. Plummer <hanetzer@startmail.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/emulation/qemu-power8/Makefile.inc

@@ -15,7 +15,6 @@
 bootblock-y += bootblock.c
 bootblock-y += uart.c
 romstage-y += cbmem.c
-ramstage-y += cbmem.c
 romstage-y += romstage.c
 ramstage-y += timer.c
 romstage-y += uart.c